1. Field of the Invention
The present invention relates to a temperature compensation circuit of an AGC (Automatic Gain Control) circuit and particularly to a temperature compensation AGC circuit which is capable of introducing a compensation voltage generating unit of the simplified structure for generating a temperature compensation voltage by adequately selecting a bias current value of a temperature compensation diode.
2. Description of the Related Art
In general, an electronic tuner comprises at least a high frequency amplifying stage for amplifying the received high frequency signal, a frequency converting stage for mixing frequencies of the amplified high frequency signal and a local oscillation signal to generate an intermediate frequency signal, an intermediate frequency amplifying stage for selectively amplifying the intermediate frequency signal, detecting (demodulating) stage for obtaining a modulated signal by detecting (demodulating) the amplified intermediate frequency signal, and a receiving signal selecting unit for selecting and setting the frequency of the local oscillation signal in order to receive the signal of desired frequency.
Moreover, even if the received high frequency signal level has changed to a large extent in such electronic tuner, an AGC (Automatic Gain Control) circuit is generally provided in view of controlling variation of the demodulated signal level output from the detecting (demodulating) stage to a comparatively small value. This AGC circuit forms an AGC voltage from the demodulated signal obtained by the detecting (demodulating) stage and supplies this AGC voltage to the high frequency amplifying stage or intermediate frequency amplifying stage. Moreover, the AGC circuit also operates to control the gain of the high frequency amplifying stage or the intermediate frequency amplifying stage with the supplied AGC voltage. Therefore, when the received high frequency signal level has changed, variation of the intermediate frequency signal level to be supplied to the detecting (demodulating) stage is compressed to a large extent.
FIG. 3 is a circuit diagram illustrating an example of the structure of AGC circuit to be used in the existing electronic tuner.
As illustrated in FIG. 3, the AGC circuit is composed of a detecting circuit unit 31, a temperature compensation circuit unit 32, and a differential amplifying unit 33. The detecting circuit unit 31 is connected, at its input end, to an intermediate frequency signal input terminal 35, to one input terminal of the differential amplifying unit 33 at its output terminal and to a power source terminal 36 at its power source supply terminal. The temperature compensation circuit unit 32 is connected to the power source terminal 36 at its power source supplying terminal and connected to the other input terminal of the differential amplifying unit 33 at its output terminal. The differential amplifying unit 33 is connected to the control terminal of the high frequency amplifying stage 34 at its output terminal. Moreover, the high frequency amplifying stage 34 is connected to the high frequency signal input terminal 37 at its input terminal, also connected to the high frequency signal output terminal 38 at its output terminal and connected to the power source terminal 39 at its power source supplying terminal.
Here, the detecting circuit unit 31 is provided with a detection diode 311, a branching capacitor 312, four resistors 313, 314, 315, 316 and a coupling capacitor 317 and these circuit elements 311, to 317, are connected as illustrated in the figure. The temperature compensation circuit unit 32 is provided with a temperature compensation diode 321, a branching-capacitor 322, four resistors 323, 324, 325, 326, a variable resistor 327 and these circuit elements 321, to 327 are connected as illustrated in FIG. 3. The differential amplifying unit 33 is provided with an operational amplifier 331, and a feedback capacitor 332 and these circuit elements 331, and 332 are connected as illustrated in FIG. 3. The high frequency amplifying stage 34 is provided with a double gate field effect transistor (FET) 341, a load inductor 342, two coupling capacitors 343, 344, resistors 345, 346, and a buffer resistor 347, and these circuit elements 341, to 347, are connected as illustrated in FIG. 3.
In this AGC circuit, a structure of the detecting circuit unit 31 is identical to a structure of the temperature compensation circuit unit 32, except for the point that a variable resistor 327 is connected to the temperature compensation circuit unit 32, and the detection diode 311 and temperature compensation diode 321 have the identical characteristics.
The AGC circuit in the structure explained above operates as explained below.
The power source voltage supplied to the power source terminal 36 is divided by the four resistors 313, 314, 315, 316 and the divided voltage is then supplied to the detection diode 311 of the detecting circuit unit 31 as a bias voltage to set the operating point of the detection diode 311. Moreover, in the temperature compensation diode 321 of the temperature compensation circuit unit 32, the power source voltage supplied to the power source terminal 36 is divided by four resistors 323, 324, 325, 326 and a variable resistor 327 and the divided voltage is then supplied to the temperature compensation diode 321 as a bias voltage to set the operating point of the temperature compensation diode 321. In this case, the operating point of the temperature compensation diode 321 can be set identical to the operating point of the detection diode 311 by adjusting the variable resistor 327.
When the intermediate frequency signal is supplied to the intermediate frequency signal input terminal 35 under the setting conditions explained above, this intermediate frequency signal is detected by the detection diode 311 of the detecting circuit unit 31 and moreover is smoothed by the branching capacitor 312. Thereafter, this intermediate frequency signal is divided by a couple of resistors 315, 316 and is then supplied to the inverted input terminal (xe2x88x92) of the operational amplifier 331 of the differential amplifying unit 33 as the first DC voltage. In addition, when the power source voltage supplied to the power source terminal 36 is also supplied to the temperature compensation circuit unit 32, this power source voltage is then divided by two resistors 323, 324 and the variable resistor 327 and is further divided by two resistors 325, 326 through the temperature compensation diode 321. Thereafter, this divided power source voltage is supplied to the non-inverted input terminal (+) of the operational amplifier 331 as the second DC voltage. The operational amplifier 33 differentially amplifies the first DC voltage and the second DC voltage supplied to two input terminals and generates the AGC voltage (positive voltage) including a differential voltage element of the first DC voltage and second DC voltage at the output terminal. This AGC voltage is then supplied to the high frequency amplifying stage 34 from the differential amplifying unit 33.
When the AGC voltage is supplied to the high frequency amplifying stage 34, it is then supplied to one gate of the double gate FET 341via the buffer resistor 347. In this case, the high frequency signal supplied to the high frequency signal input terminal 37 is then supplied to the other gate of the double gate FET 341 via the coupling capacitor 343 and is then amplified by the double gate FET 341. The amplified high frequency signal is then supplied to the high frequency signal output terminal 37 via the coupling capacitor 344.
Here, when the high frequency signal level to be input to the high frequency amplifying stage 34 becomes high, the high frequency signal level output from the high frequency amplifying stage 34 also becomes high and simultaneously the intermediate frequency signal level to be input to the detecting circuit 31 also becomes high, resulting in increase of the first DC voltage to be supplied to the inverted input terminal (xe2x88x92) of the operational amplifier 331. However, since the second DC voltage to be supplied to the non-inverted input terminal (+) of the operational amplifier 331 is constant, the AGC voltage output from the operational amplifier 331 is reduced in its positive voltage element and therefore substantially becomes low level. When this low level AGC voltage is supplied to the double gate FET 341 of the high frequency amplifying stage 34, signal gain of the double gate FET 341 is lowered as much as the reduction of the AGC voltage. Therefore, the high frequency signal level output from the high frequency amplifying stage 34 is also lowered.
Meanwhile, when the high frequency signal level to be input to the high frequency amplifying stage 34 is lowered, the high frequency signal level output from the high frequency amplifying stage 34 is also lowered. As a result, the intermediate frequency signal level to be input to the detecting circuit unit 31 is also reduced and thereby the first DC voltage to be supplied to the inverted input terminal (xe2x88x92) of the operational amplifier 331 is lowered and the positive element of the AGC voltage output from the operational amplifier 331 is increased and substantially becomes large. When this large AGC voltage is supplied to the double gate FET 341 of the high frequency amplifying stage 34, signal gain of the double gate FET 341 in increased as much as increase of the AGC voltage and thereby the high frequency signal level output from the high frequency amplifying stage 34 is also increased.
As explained above, when the high frequency signal level to be input to the high frequency amplifying stage 34 is increased, gain of the high frequency amplifying stage 34 is reduced by the AGC voltage and when the high frequency signal level to be input to the high frequency amplifying stage 34 is reduced, gain of the high frequency amplifying stage 34 is increased by the AGC voltage to realize the predetermined AGC operation.
The reason why the detecting circuit unit 31 and temperature compensation circuit unit 32 of almost identical structure are used and the detection diode 311 and temperature compensation diode 321 are used in the AGC circuit of the related art is that since the detection diode 311 has the temperature characteristic, if the temperature compensation circuit unit 32 is not used, the AGC voltage varies for the change of ambient temperature but change of AGC voltage for variation of ambient temperature is canceled due to the output voltage of the temperature compensation circuit unit 32 provided with the temperature compensation diode 321 having the characteristic identical to that of the detection diode 311 in view of attaining the constant AGC voltage for variation of ambient temperature.
The AGC circuit of the related art uses the temperature compensation circuit unit of the structure identical to that of the detection circuit unit 31 to conduct the predetermined temperature compensation by controlling variation of the AGC voltage for the change of ambient temperature. However, since the temperature compensation circuit unit 32 is required to have the structure identical to the detection circuit unit 31, it is impossible to avoid that the circuit configuration of the temperature compensation circuit unit 32 is comparatively complicated.
Moreover, the AGC circuit of the related art conducts the predetermined temperature compensation by controlling the change of AGC voltage for variation of ambient temperature but cannot conduct temperature compensation even for gain characteristic of the high frequency amplifying stage which varies for change of ambient temperature. As a result, gain variation of the high frequency amplifying stage for change of the ambient temperature cannot be eliminated.
The present invention has been proposed considering the technical background as explained above and it is therefore an object of the present invention to provide a temperature compensation AGC circuit which simplifies the circuit configuration of the temperature compensation circuit unit and eliminates variation of gain characteristic of the stage to be controlled in its gain for change of ambient temperature.
In order to achieve the object explained above, the temperature compensation AGC circuit of the present invention comprises a signal detecting unit including the detection diode to generate a detection voltage in proportion to the signal level, a compensation voltage generating unit including the temperature compensation diode to generate temperature compensation voltage, and a differential amplifying unit to generate the AGC voltage from a difference voltage between the detection voltage and temperature compensation voltage and to supply such AGC voltage to a gain amplifying stage to be controlled. By giving the temperature characteristic to the AGC voltage through given difference between the bias current flowing through the detection diode and the bias current flowing through the temperature compensation diode, change of gain of the gain amplifying stage to be controlled depending on change of ambient temperature is controlled.
As a preferable example of the means explained above, the compensation voltage generating unit is composed of the temperature compensation diode and a variable voltage dividing circuit to supply a bias current to this temperature compensation diode.
As another preferable example of the means explained above, the detection diode and the temperature compensation diode have the identical temperature characteristic.
As another preferable example of the means explained above, the bias current flowing into the temperature compensation diode is set lower than the bias current flowing into the detection diode.
In above means, the AGC voltage which changes a little for change of ambient temperature and this AGC voltage is supplied to the amplifying stage to be controlled in its gain in order to control change of gain of the amplifying stage to be controlled in its gain for change of ambient temperature by simplifying the circuit configuration of the compensation voltage generating unit and giving difference between the bias current values flowing into the detection diode and temperature compensation diode, namely giving difference between the operation points of the temperature compensation diode and detection diode.
As explained above, according to this means, it is now possible to obtain the AGC circuit which not only simplifies the circuit configuration of compensation voltage generating unit but also assure sufficient temperature compensation for change of ambient temperature.